1. Field of the Invention
This invention relates to a video signal reproducing apparatus and more particularly to a video signal reproducing apparatus having a memory device which is capable of storing a portion of the video signal covering a predetermined period of time.
2. Description of the Related Art
FIG. 1 of the accompanying drawings shows the heads of the conventional video tape recorder (hereinafter referred to as VTR) which is a reproducing apparatus of the above stated kind as in relation to recording tracks formed on a record bearing medium and also shows the reproduction outputs of the heads.
The conventional method for improving picture quality during the so-called special reproduction and particularly during a high speed reproducing search made for a video signal to be reproduced by allowing the tape to travel at a high speed has been as follows: Referring to FIG. 1, two heads of different azimuth angles are adjacently arranged as shown in FIG. 1. The output of each of the heads is arranged to be taken out only when the head is tracing a track of the same azimuth angle as the head. The method gives a picture with inconspicuous noise bars. However, in the event of a two-head helical scanning type VTR, this method necessitates use of at least four heads. Besides, in that event, two of these heads must be adjacently arranged. This results in a complex head arrangement. Further, the number of slots of a rotary transmitter increases according as the number of heads increases thus resulting in an increase in the size of the rotary transmitter. In cases where the VTR must use a compact rotary head type drum like in the case of the so-called 8 mm VTR, therefore, the above stated method has been hardly employable.
Further, in an effort to make reproduced images more easily discernible in performing high speed search reproduction with the VTR of the above stated kind arranged to have two heads, it has been practiced to have noise bars appear in a fixed part on the picture plane.
FIGS. 2(a) and 3(a) show a tape which is employed as a record bearing medium and the tracing loci of the heads obtained on the tape under high speed search reproduction performed by the conventional VTR which is arranged to do tracking control in the so-called four frequency method. In these drawings, reference symbols f1, f2, - - - denote pilot signals recorded on the tape. Reference symbols Ai, Bi, - - - (wherein "i" represents one of integers 1, 2, - - - ) denote the tracing loci of the heads A and B which are of different azimuth angles.
(i) High speed search reproduction at a tape travel speed (4m+1) times as high as the tape speed employed for recording is performed as follows: FIG. 2(a) shows the tracing loci of the rotary head obtained on the tape in case that the tape speed is determined with "m" set at m=1. Symbols f1, f2, f3 and f4 denote the frequencies of the pilot signals which are recorded in recording tracks in a superimposed manner. FIG. 2(b) shows the frequencies of reference pilot signals by which reproduced pilot signals are to be multiplied at a tracking control circuit arranged to operate in accordance with the known four-frequency method.
In obtaining a tracking error signal by the known four-frequency method, a frequency component fA (=f2-f1=f3-f4) which is included in a multiplication output obtained from the reproduced pilot signal and the reference pilot signal is compared with a frequency component fa (=f4-f1=f3-f2). The result of comparison is used as the tracking error signal (hereinafter referred to as ATF signal). This error signal represents a tracking error relative to a recording track in which the pilot signal of the same frequency as that of the reference pilot signal is recorded. Therefore, in the case of the high speed search reproduction at a tape speed (4m+1) times as high as the recording tape speed, a track to be controlled appears at intervals of (4m+1) tracks. Accordingly, the frequencies of the reference pilot signals are changed from one over to another by rotation in the order of f1 - f2 - f3 - f4 every time the rotary head traces the tape. The ATF signal which is thus obtained is sampled and held at a given phase of the rotary head. A tracking control signal then can be obtained from this for making the noise bar appearing part unvarying. Further, the tracing loci shown in FIG. 2(a) is obtained by sampling and holding the ATF signal immediately after the rotary head comes to the tape.
(ii) High speed search reproduction at a tape speed (4m+3) times as high as the tape speed employed in recording is performed as follows: FIG. 3(a) shows tracing loci obtained with the tape speed determined by setting "m" at m=1. FIG. 3(b) shows the frequencies of the reference pilot signals. In this instance, the reference pilot signals are arranged to be produced by rotation in the frequency order of f1 - f4 - f3 - f2. Then, in the same manner as in the case of FIG. 2, the noise bar appearing part can be made unvarying by sampling and holding the ATF signal at a given phase of the rotary head. The tracing loci in this instance is also obtained by sampling and holding the ATF signal immediately after the rotary head comes to the tape in the same manner as in the case of FIG. 2(a).
The noise bar appearing part thus can be fixed during the high speed search reproduction in this manner. However, the conventional method is incapable of preventing the disagreeable noise bars from being generated.
The special reproducing operations which are to be accomplished at tape (moving or travel) speed differing from the tape speed employed in recording include, for example, still picture reproduction which is to be made at a tape speed 0 times as high as the recording tape speed and high speed reproduction which is to be made at a tape speed n times (n&gt;1) as high as the recording tape speed and a slow reproduction which is to be made at a tape speed 1/n times as high as the recording tape speed. The VTR of the type having such varied speed reproducing functions has been arranged in varied improving attempts to lessen or make less conspicuous the appearance of noises on a reproduced picture plane resulting from reproduction of the record with the rotary heads straddling more than two tracks of different azimuth angles in tracing them like in the case of the above stated example of the conventional VTR. In addition to these noise suppressing attempts, efforts have been exerted to make improvement also in other points.
One of other points of improvement is as follows: Since a relative speed obtained between the tape and the head during reproduction differs from the relative speed obtained in recording, color deviation, etc. take place to deteriorate the picture quality on the picture plane. The picture quality deterioration must be prevented. Another point of improvement is as follows: Since, in this instance, the tape speed differs from the normal tape speed, the VTR must be arranged to prevent the continuity of a vertical synchronizing signal from being impaired by the straddling timing of the rotary head from one track over to another in reproducing the record. In that instance, one frame portion of the video signal is arranged to be formed with two field portions of the video signal by offsetting the vertical synchronizing signals of odd- and even-number fields.
In attaining the former point of improvement, the number of the rotational frequency of a drum motor which rotates the rotary head is arranged to be variable in such a manner as to have the relative speed which obtains between the head and the tape during the varied speed reproduction approximately coincide with the relative speed obtained in recording. In the case of the latter point of improvement, since the head in general traces the tape by straddling at least two adjacent tracks in the event of variable speed reproduction, the continuity of the vertical synchronizing signal is arranged to be retained by inserting later a dummy vertical synchronizing signal; and, at the same time, offset adjustment is made between the fields. Furthermore, in connection with the former point, the rotational frequency of the rotary head is arranged to be different from the rotational frequency of the rotary head employed in recording or in normal reproduction for the purpose of having the varied speed reproduction performed at the same relative speed between the head and the tape as in recording. The dummy vertical synchronizing signal is then formed on the basis of rotation phase detection pulses representing the rotation phase of the rotary head according to the above stated different rotational frequency.
Meanwhile, a VTR of the type arranged to use a field memory device in the above stated mode of varied speed reproduction has appeared during the recent years. According to the arrangement of this type of VTR, one field portion of a video signal having no noise or not much noise is written as data into this field memory. Then, the written data is continuously read out as necessary. More specifically, the VTR having the field memory has one field portion of a reproduced video signal which gives a sufficiently large reproduction output stored at the memory in the case of, for example, still picture reproduction and then repeatedly read out as an output for the same one field portion of the video signal. Therefore, in that instance, one and the same field portion of the reproduced video signal is repeatedly produced as a reproduced signal. Under this condition, the odd- and even-number fields have the vertical synchronizing signal at the same phase. Therefore, for the even-number field, the phase of the vertical signal must be shifted to an extent corresponding to 1/2 H (H: horizontal scanning period) by inserting a dummy vertical synchronizing signal.
FIG. 4 shows in a block diagram the arrangement of the essential parts of the conventional VTR having a field memory device which is arranged to be used at the time of still picture reproduction. FIG. 5 shows in a block diagram the conventional drum servo circuit which is arranged to rotate a rotary head. Referring to FIGS. 4 and 5, a synchronizing signal separation circuit 2 is arranged to separate synchronizing signals from a reproduced video signal which comes via a terminal 1. The VTR comprises a clock signal generator 3; an analog-to-digital (A/D) converter 4; an address control circuit 5; a field memory 6; reference clock signal generators 7 and 10; a digital-to-analog (D/A) converter 8; a frequency divider 11; a trapezoidal wave forming circuit 12; a pulse forming circuit 13; a frequency multiplier 14; a sample and-hold (S/H) circuit 15; a speed servo circuit 16; an adder 17; a motor driving circuit 18; and a motor 19.
Referring to FIG. 4, a reproduced signal produced from a rotary head which is not shown undergoes a known signal processing operation to be converted into a composite television signal. The composite signal is supplied to the input terminal 1. The synchronizing signal separation circuit 2 then separates the synchronizing signal from this composite signal. The clock signal generator 3 is caused to oscillate by the separated signal in synchronism with the latter. The A/D converter 4 samples the composite signal according to the output of the clock signal generator 3 and converts the composite signal into a digital signal. In this instance, the sampling frequency is approximately set at 4 fsc (fsc: the color subcarrier frequency of the chrominance signal). However, in case that the separated synchronizing signal has some time-base error, the sampling frequency varies according to the time base error.
Meanwhile, the output of the clock signal generator 3 is supplied also to the address control circuit 5 for controlling the writing address of the field memory 6. The above stated digital data is stored at the writing addresses of the field memory 6 in sequence as determined by the control circuit 5.
In reading out the stored digital data, reading addresses are controlled by a clock signal produced from the reference clock signal generator 7 which is arranged, for example, to generate clock pulses at the frequency of 4 fsc. The digital data thus read out from the addresses one after another is supplied to the D/A converter 8 which is driven to operate also by the reference clock signal from the generator 7. The D/A converter 8 produces an analog signal which resumes the form of the composite television signal. The composite signal is then produced from an output terminal 9.
FIGS. 6(a) to 6(d) show in a timing chart the wave forms of the outputs of various parts of the drum servo circuit of FIG. 5. The clock signal of the frequency 4 fsc produced from the reference clock signal generator 10 is frequency divided by the frequency divider 11 which is arranged to perform frequency division of 1/238848. The frequency divider 11 then produces a clock signal of 60 Hz as shown in FIG. 6(a). This clock signal is supplied to the trapezoidal wave forming circuit 12. The circuit 12 then produces an output which is as shown in FIG. 6(b). The S/H circuit 15 samples and holds the output of the circuit 12 in accordance with pulses produced from the frequency multiplier 14. A PG head which is arranged to detect the rotation phase of the drum motor 19 produces a rotation detecting pulse signal (hereinafter referred to as PG pulses). The PG pulses are supplied to the pulse forming circuit 13 to be converted into a pulse signal of 30 Hz as shown in FIG. 6(c). The pulse signal is supplied to the frequency multiplier 14 to have its frequency multiplied by two as shown in FIG. 6(d) and thus becomes sampling pulse signal to be applied to the S/H circuit 15. The output of the S/H circuit 15 serves as a phase error signal. The adder 17 adds this phase error signal and a speed error signal coming from the speed servo circuit 16. The result of addition is applied to the drum motor driving circuit 18. The speed servo circuit 16 is arranged to obtain by a known method the above stated speed error signal representing the frequency speed error of a signal produced from an FG head relative to the rotating speed of the drum motor 19 and to supply it to the adder 17.
In performing varied speed reproduction with the conventional VTR using the field memory, when the rotating speed of the rotary head is changed for the purpose of making the relative speed between the tape and the head the same as the relative speed obtained at the time of recording, the period during which one field portion of the video signal is reproduced by the rotary head does not coincide with a period required for reading out one field portion of the video signal from the field memory. Therefore, the vertical synchronising timing of the signal read out from the field memory deviates from the signal reproduced by the rotary head during the varied speed reproduction. As a result, the reproduced picture is disturbed by the timing deviation. Meanwhile, an attempt to avoid this by adjusting the rotating speed of the rotary head to the head rotating speed employed in recording would result in the color deviation of the reproduced picture as mentioned in the foregoing. Further, the effort to obtain one field portion of a video signal in a relatively good state as mentioned in the foregoing does not much improve the quality of a reproduced picture in the event of, for example, a high speed search reproducing operation or the like, because many tracks are to be traversed by the head while one field portion of the video signal is reproduced in that instance.